This invention relates to lightning surge arresters, and more particularly to improvements in such arresters which include non-linear resistors constructed of metal oxide, for example, zinc-oxide.
Lightning arresters are protective devices used to limit surge voltages in equipment caused by lightning or other disturbances in the equipment circuit. Lightning arresters generally function to bypass or discharge surge current and discharge surge voltages within a fraction of a cycle and prevent damage to the protected equipment.
Many prior lightning arresters use non-linear resistors which are isolated from the line voltage by spark gaps in series with the resistors. The gaps prevent current flow to the resistors under normal conditions. However, when a surge occurs in the line, "spark-over" or "arcing" occurs across the gap and the surge voltage and surge current are discharged. The gap dimension establishes the surge voltage at which arcing occurs.
One of the problems which arises in using arresters having arcing gaps in series with non-linear resistors is that contamination can trigger repetitive spark-over which causes the arrester to overheat. Also, arresters having arcing gaps in series with the non-linear resistors have an inherent time lag before arcing occurs. At surge conditions, this means that the voltage can rise to a dangerous level before it is discharged.
Non-linear resistors having very good non-linear characteristics have recently been developed. These include resistors constructed of zinc oxide, and, because of their excellent non-linearity, they make it possible to eliminate arcing gaps in series with the resistors in the arresters. By eliminating the series gaps, there is no time lag before the arrester goes into conduction and the surge voltage is kept to a low level. Also, contamination does not have the undesirable effect it does in arresters having series gaps.
Although it is desirable to eliminate series gaps in arresters, it may be desirable to employ an arcing gap device in parallel with a portion of the zinc oxide resistors. Such an arrangement shunts a portion of the zinc oxide resistors and improves the level of protection provided by the arrester. The arcing level of such parallel gaps must be above the normal operating voltage but below the arrester voltage when the protective voltage level is reached.
Many prior arresters using non-linear resistors, including those of zinc oxide, and a parallel arcing gap device are designed to have a prescribed rated voltage, i.e., 14 KV. If an installation requires a higher voltage rating, two or more arrester units can be connected together. The arresters include an outer housing enclosing the zinc oxide resistors and the parallel gap device, and can contain nitrogen or other gas, or they can be sealed and the air evacuated.
The arrester housing is desirably constructed of metal which can be grounded so that the arrestor can be placed as closely as possible to the equipment to be protected. In such cases, the zinc oxide resistors must be insulated from the metal housing. To achieve this required insulation effect but with minimal space between the resistors and metal housing and therefore minimal overall arrester size, it has been proposed to fill the housing with sulfur hexafluoride gas (SF.sub.6) because of its excellent electrical insulating qualities. This is particularly advantageous in arrestors used in miniaturized electric substations, metal clad switch gears, and the like, where overall size of the arrester is critical.
It will be appreciated that when sulfur hexafluoride gas is used in the arrestor housing, its excellent insulation characteristics also requires that the dimension of the parallel arcing gap be reduced. However, it is necessary that the parallel gap device be stable in operation and that spark-over occur at the desired predetermined voltage. If the arcing gap becomes too small, stability is difficult to maintain.